Figure 1.
Experimental setup and protocol.
Subjects stood on a balance board with a narrow beam of support and performed forward leans to move a cursor via their center of pressure to a target presented on a LCD screen. To add uncertainty, targets may jump forwards, backwards, or remain at the original target position. A top down view of the balance board and feet illustrates that stability margins represent the difference between the width of the beam and the range of medio-lateral center of pressure (CoP) excursion. The experimental protocol consisted of 200 null trials, 300 target-jump trials, and 100 post trials. During the target-jump block, each of the three possible target-jump distances were presented 100 times in a randomized order. Key time points for comparison were Late Null (diagonal hatch), Early Uncertainty (gray fill), and Late Uncertainty (black fill).
Figure 2.
Uncertainty promotes maneuverability.
Group averaged (n = 11) anterior-posterior (AP) center of pressure time series profiles for early (light gray) and late (dark gray) backwards target-jumps. Solid lines are group means and shaded areas are±s.d. Subjects had shorter response times at late versus early target-jump. The maneuverability metric was the response time required for subjects to reverse center of pressure direction in response to a mid-movement backwards target-jump. Bars are the group means±s.e.m. response times for each time point.
Figure 3.
Maneuverability comes at the expense of stability.
Group averaged (n = 11) medio-lateral (ML) center of pressure time series profiles for late null (light gray) and late target-jump (dark gray). These conditions compared trials to the same target distance. Solid lines are group means and shaded areas are±s.d. The stability metric was the width of the stability margins which were larger for late null (diagonal hatch) compared to late target-jump (dark gray). Bars are the group means±s.e.m. of the stability margin for each time point.
Table 1.
Center of pressure (CoP) measures (mean±s.d.) for all phases of the experiment.
Figure 4.
The percent of success and stability.
Percent of successful trials in 0.1 cm bins of stability margin across the width of the beam of support. Thick line is the mean and the shaded area is±s.d. Larger stability margins corresponded with higher probabilities of success and implied increased stability. Smaller stability margins corresponded with lower probabilities of success and implied decreased stability. These data support the definition of stability as the probability of keeping the board level and of stability margin as a metric of stability.
Figure 5.
Stability-maneuverability tradeoff for individual subjects.
Change in response times (early and late jump-b) versus change in stability margins (late null and late jump-0) for each individual subject. A positive change in response time indicated a faster response time by late target-jump. A positive change in stability margin indicated larger stability margins by late target-jump. Four quadrants characterize changes in movement strategy to be 1) more maneuverable/more stable, 2) more maneuverable/less stable, 3) less maneuverable/less stable, and 4) less maneuverable/more stable. Circles represent subjects (n = 8) who exhibited a stability-maneuverability tradeoff whereas X's represent subjects (n = 3) who did not exhibit the tradeoff.
Figure 6.
Group averaged muscle activity linear envelopes for late null and late jump-0.
Linear envelopes were similar between late null (thin line) and late jump-0 (thick line). The dashed vertical line indicates when the cursor moved out of the home circle.
Figure 7.
Group averaged RMS EMG amplitudes for all 16 lower limb muscles.
Thick lines are the narrow beam data while dotted lines are the wide beam data. Left muscles are black and right muscles are gray. Single asterisks indicate a significant increase in left muscle activity compared to the right muscle on the narrow board. Black double asterisks indicate a significant increase in muscle activity in the left muscle on the narrow beam compared to the wide beam board, while gray double asterisks indicate a significant increase in muscle activity in the right muscle. Error bars are standard error of the mean.
Figure 8.
Schematic of coactivation definition, coactivation linear envelopes, and group averaged RMS coactivation amplitudes.
A) Coactivation was the minimum value of EMG1 and EMG2. B) Coactivation linear envelopes between late null (thin line) and late jump-0 (thick line) were similar. The dashed vertical line indicates when the cursor moved out of the home circle. C) Thick lines are the narrow beam data while dotted lines are the wide beam data. Late null and late jump-0 are in black. Early and late jump-b are gray. Black double asterisks indicate a significant increase in coactivation in the lateral muscle pairs on the narrow beam compared to the wide beam board during late null to late jump-0. Similarly, gray double asterisks indicate a significant increase in coactivation on the narrow beam during early to late jump-b. Error bars are standard error of the mean.